To calculate the number of bags of concrete you’ll need for a 4×5 slab, you’ll first need to determine the desired thickness of the slab. A typical thickness for a concrete slab is 4 inches (0.333 feet), but you can adjust this based on your specific project requirements.

Assuming you want a 4-inch thick slab for your 4×6 area, you can calculate the volume of concrete needed as follows:

1. Calculate the volume in cubic feet:

Volume = Length (in feet) x Width (in feet) x Thickness (in feet). Volume = 4 x 5 x 0.333 = 6.66 cubic feet.

2. Next, you’ll need to convert this volume to the number of bags of concrete. A typical 80-pound bag of concrete mix covers about 0.6 cubic feet when mixed with water.

3. Divide the total volume (6.66 cubic feet) by the volume per bag (0.6 cubic feet per bag): 6.66 cubic feet / 0.6 cubic feet per bag ≈ 11.1 bags.

Since you can’t buy a fraction of a bag, you’ll need to round up to the nearest whole number. So, you’ll need approximately 11 bags of concrete for a 4×5 slab that is 4 inches thick.

A bag of concrete ready mix made of blended mix of Portland cement, sand gravel and other approved ingredient or admixture, generally, dry ready mix concrete are available in different bag size and weight condition such as 40lb, 60lb bag, 80lb bag, 90lb bag and etc.

Regardless of size you need to calculate the volume of concrete in cubic yard or in cubic feet or in cubic meter or in rmc truck before ordering the mixed concrete. Volume is calculated by multiplying length, width and depth of slab. Here you have given a slab of 4 feet in length and 5 feet in wide, what is thickness of slab?, it’s depend on, is this a slab on the ground or it is elevated, rules differ for both, since failure of ground slab would not kill anyone whereas failure of an elevated slab could.

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Thumb rules involves the safety factor that deciding the thickness of slab, 4 inch thick is generally for ground slab, heavy traffic and heavy load, you should take depth 5 or 6 inch thick and for an elevated slab generally thickness should be 5″ to 6″, and 4 inch thick is also made to use in normal conditions.

Generally standard thickness of slab is 4 inch thick in normal conditions, if 4×5 slab have heavy traffic and heavy load, you should take depth 5 or 6 inch thick. In this article we have to calculate required quantity of concrete in cubic yard & in cubic feet and cubic meter for a 4×5 (20 sq ft) slab at 4 inch, 5 inch or 6 inch thick.

Weight of Ready mix concrete in dry condition is about 133 lbs per cubic foot, 3600 lbs per cubic yard, or approximately equal as 2136 kg per cubic meter. The weight of concrete is determined by its density, which can vary based on the amount of aggregate, water, and air in the mix.

In United States, based on Imperial and US customary measurement system, 1 cubic yard is unit of measurement of volume represented by 3 feet long by 3 feet wide by 3 feet depth (3’×3’× 3′ = 27 cubic feet), so 1 cubic yard is equal to 27 cubic feet.

**How many bags of concrete do I need for a 4×5 slab**

To determine the how much concrete do I need for a 4×5 slab & how many bags of concrete do I need for this, you can solve in following steps:-

**Step 1 :-** calculate the volume in cubic feet:- you have given 4×5 slab (length × width), consider it is 4 inch thick, 4 inch = 0.33 feet, to determine the volume of slab, you need to multiply all dimensions, such that = 4′×5′× 0.33′ = 6.6 cubic foot, at 5 inch thick, volume of slab is 4′×5′× 0.416 = 8.32 cubic foot and at 6 inch thick, volume of slab is 4′×5′× 0.5 = 10 cubic foot.

**Step 2 :-** converting cubic feet into cubic yard:- now converting into cubic yard, you should divide it by 27, so volume of concrete you needed for 4×5 slab at 4 inch thick in cubic yard = 6.66/27 = 0.244 cubic yards, at 5 inch thick = 8.32/27 = 0.308 cubic yards & at at 6 inch thick = 10/27 = 0.37 cubic yards.

**Step 3:-** add 10% extra which would be wastage during mixing and pouring.

**Step 4:-** to determine how many bags of concrete you will need, divide the total cubic yards needed by the yields, as you know premixed bag of concrete available in 40lb, 60lb & 80lb. 40lb bag of concrete yields about 0.011 cubic yards, 60lb bag of concrete yields about 0.017 cubic yards and 80lb bag of concrete yields about 0.022 cubic yards.

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**How many yards of concrete do I need for a 40×60 slab**

**Number of bags of concrete for a 4×5 slab at 4 inch thick**

To determine how many bags of concrete you will need, divide the total cubic yards needed by the yields, at 4 inch thick, for a 4×5 slab you will need 0.244 yards of concrete, so number of 80lb bag of concrete = 0.244 ÷ 0.022 = 11 bags, No. of 60lb bag = 0.244 ÷ 0.017 = 15 bags & No. of 40lb bag = 0.244 ÷ 0.011 = 22 bags.

**Number of bags of concrete for a 4×5 slab at 5 inch thick**

To determine how many bags of concrete you will need, divide the total cubic yards needed by the yields, at 5 inch thick, for a 4×5 slab you will need 0.308 yards of concrete, so number of 80lb bag of concrete = 0.308 ÷ 0.022 = 14 bags, No. of 60lb bag = 0.308 ÷ 0.017 = 18 bags and No. of 40lb bag = 0.308 ÷ 0.011 = 28 bags.

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**Number of bags of concrete for a 4×5 slab at 6 inch thick**

To determine how many bags of concrete you will need, divide the total cubic yards needed by the yields, at 6 inch thick, for a 4×5 slab you will need 0.37 yards of concrete, so number of 80lb bag of concrete = 0.37 ÷ 0.022 = 17 bags, No. of 60lb bag = 0.37 ÷ 0.017 = 22 bags and No. of 40lb bag = 0.37 ÷ 0.011 = 34 bags.

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Regarding this, **“how many bags of concrete do I need for a 4×5 slab?”**, at 4 inches thick, generally you will need approximately either 15 bags of 60lb or 11 bags of 80lb of premixed concrete for a 4×5 slab, at 5 inches thick slab, either 18 bags of 60lb or 14 bags of 80lb premixed concrete are required, while at 6 inches thick slab, either 22 bags of 60lb or 17 bags of 80lb premixed concrete is necessary.

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**How many yards of concrete do I need for a 30×40 slab**

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**How many yards of concrete do I need for a 20×40 slab**

**Conclusions**:-

As per calculation, for a 4×5 slab at 4 inch thick, generally you will need approximately either 15 bags of 60lb or 11 bags of 80lb of premixed concrete, at 5 inches thick slab, either 18 bags of 60lb or 14 bags of 80lb premixed concrete are required, while at 6 inches thick slab, either 22 bags of 60lb or 17 bags of 80lb premixed concrete is necessary.